The present invention relates to a linear amplifier for use mainly in the high-frequency band and, more particularly, to a feedforward amplifier with dual loop, which is provided with a distortion detection circuit for detecting distortion produced by a main amplifier, and a distortion elimination circuit which amplifies the detected distortion component by a first auxiliary amplifier and injects the amplified component into the output from the main amplifier to thereby cancel the distortion component and wherein the distortion elimination circuit comprises a distortion detection circuit which detects a distortion component produced by the first amplifier and a distortion elimination circuit which cancels the detected distortion component by its injection into the first auxiliary amplifier output.
A radio base station for mobile communications employs a feedforward amplifier in a transmitting power amplifier. The feedforward amplifier is composed basically of two signal cancellation circuits, one of which is a distortion detection circuit made up of a signal transfer path of a main amplifier and a linear signal transfer path, and the other of which is a distortion elimination circuit made up of a main signal transfer path and a distortion injection path. The linear signal transfer path is formed by a delay line and a phase inverter circuit. The main signal transfer path is formed by a delay line. The distortion injection path is formed by a variable attenuator, a variable phase shifter and an auxiliary amplifier.
The input signal to the feedforward amplifier is divided by a power divider circuit to the signal transfer path and the linear signal transfer path of the main amplifier. The output signal from the feedforward amplifier is provided by a power combiner which combines outputs from the main signal transfer path and the distortion injecting path. With such a feedforward amplifier, a nonlinear distortion component produced by the main amplifier are detected by the distortion detection circuit and the detected distortion component are eliminated by being injected into the main amplifier output path so that the distortion component and the main amplifier output are opposite in phase but equal in amplitude and delay.
With the recent rapid proliferation of mobile communication, there is now a demand for small, low-power consumption radio equipment for base station use. The base-station radio equipment comprises a modem, a transmitting power amplifier, an antenna, etc. The reduction of the power consumption of the transmitting power amplifier is effective in miniaturization of the radio equipment. To reduce the power consumption of the transmitting power amplifier, it is indispensable to increase the efficiency of the feedforward amplifier. The attainment of high efficiency for the feedforward amplifier requires to increase the efficiencies of the main amplifier and the auxiliary amplifier. The increased efficiency of individual amplifier circuits permits reduction of the power consumption of amplifier. This allows miniaturization of the cooling body of each amplifier and, as a result, enables reduction of the power consumption and downsizing of the transmitting power amplifier.
The efficiency of the main amplifier can be increased using a high-efficiency amplifier such as a class-B push-pull amplifier. In general, when a semiconductor amplifying element of the amplifier is operated under the class-B bias condition, the linearity of the circuit is poorer than under the class-A bias condition. As referred to above, however, the nonlinear distortion component resulting from the application of the class-B bias to the main amplifier of the feedforward amplifier can be eliminated by the conventional feedforward configuration.
On the other hand, to enhance the power efficiency of the auxiliary amplifier inserted in the distortion injection path of the feedforward amplifier, it is necessary, in general, that the semiconductor amplifying element of the auxiliary amplifier be operated under the class-C bias condition. With the above-described feedforward configuration, however, it is impossible to eliminate a nonlinear distortion component produced by the auxiliary amplifier. A solution to this problem is to utilize the feedforward configuration for the distortion injection path including the auxiliary amplifier.
More specifically, the distortion injection path for the auxiliary amplifier is formed by the distortion detection circuit and the distortion elimination circuit, regarding the auxiliary amplifier as a main amplifier. The nonlinear distortion component produced by the auxiliary amplifier is detected by the auxiliary-amplifier distortion detection circuit, and the detected distortion component is eliminated by the auxiliary-amplifier distortion elimination circuit. With this scheme, it is possible to apply to the semiconductor amplifying element of the auxiliary amplifier a high-efficiency-amplification-enabling bias condition other than the class-A bias condition.
FIG. 1 illustrates in block form the conventional feedforward amplifier disclosed in Japanese Patent Application Laid Open Gazette 2000-286645 (corresponding U.S. Pat. No. 6,320,461. The illustrated feedforward amplifier is made up of a distortion detection circuit 10 for detecting a distortion component produced by a main amplifier 14 and a distortion elimination circuit 50 for eliminating the detected distortion component.
The distortion detection circuit 10 is formed by a signal transfer path 10A of the main amplifier 14 and a linear signal transfer path 10B. The input signal to the input terminal 8 of the feedforward amplifier is divided by a power divider 11 to the main amplifier signal transfer path 10A including a variable attenuator 12, a variable phase shifter 13 and the main amplifier 14, and to the linear signal transfer path 10B made up of a delay line 15 and a phase inverter circuit 16. The outputs from these two paths 10A and 10B are combined and then divided by a power combiner/divider 17. The divided signals are provided to a main signal transfer path 10C and a distortion injection path 10D which constitute the distortion elimination circuit 50.
The main signal transfer path 10C is formed by a delay line 51. The distortion injection path 10D is comprised of a first auxiliary amplifier distortion detection circuit 60 for detecting a distortion component produced by a first auxiliary amplifier 63, and a first auxiliary amplifier distortion elimination circuit 70 for injecting the detected distortion component into the first auxiliary amplifier output in such a manner as to be opposite in phase but equal in amplitude and delay to each other as referred to previously.
The first auxiliary amplifier distortion detection circuit 60 comprises a first auxiliary amplifier signal transfer path 16E including a variable attenuator 61, a variable phase shifter 62 and the first auxiliary amplifier 63, and a first auxiliary amplifier linear signal transfer path 16F including a delay line 64 and a phase inverter circuit 65. The outputs from these two paths 16E and 16F are combined and then divided by a power combiner/divider 66.
The first auxiliary amplifier distortion elimination circuit 70 comprises a first auxiliary amplifier main signal transfer path 17G formed by a delay line 71, and a first auxiliary amplifier distortion injection path 17H including a variable attenuator 72, a variable phase shifter 73 and a second auxiliary amplifier 74. The outputs from these two paths 17G and 17H are combined by a power combiner 76. The distortion elimination circuit 50 combines the outputs from the main signal transfer path 10C and the distortion injection path 10D by a power combiner 53 to eliminate the distortion component produced by the main amplifier 14, and the combined signal is output as an output signal of the feedforward amplifier to the output terminal 9.
The auxiliary amplifier in the distortion injection path 10D, formed as feedforward amplifier, needs to balance each of four loops; that is, it is necessary to balance the loop of the distortion detection circuit 10, the loop of the first auxiliary amplifier distortion detection circuit 60, the loop of the first auxiliary amplifier distortion elimination circuit 70 and the loop of the distortion elimination circuit 50.
The four loops are respectively required to control sets of variable attenuators and variable phase shifters (12, 13), (61, 62) and (72, 73) so that the output signals from the amplifier signal transfer path and the linear signal transfer path become equal in amplitude and in delay and opposite in phase to each other and so that the output signals from the main signal transfer path and the distortion injection path become equal in amplitude and in delay and opposite in phase to each other. In general, improvement in the nonlinear distortion of the feedforward amplifier depends on the equilibrium of the loops by the adjustment of the variable attenuators and the variable phase shifters. The accuracy of adjustment is described in Japanese Patent Publication Gazette No. 7-77330 entitled “Automatic Adjustment Circuit for Feedforward Amplifier.” For example, phase and amplitude deviations (or differences) for providing an amount of distortion compression over 30 dB are within ±2° and within ±0.3 dB, respectively, from which it is seen that precise conditions are imposed on the degree of balance of transmission characteristics and completeness of adjustment of the distortion detection circuit 10 and the distortion elimination circuit 50. In practice, it is not easy to completely maintain the balance of the respective circuits with the distortion detection circuit 10 and the distortion elimination circuit 50. Further, even if initialized perfectly, amplifier characteristics change with variations in ambient temperature, power supply and so forth, making it very difficult to stably maintain the above-mentioned circuits on well-balanced condition for a long period of time.
As a method for maintaining the distortion detection circuit and the distortion elimination circuit of the feedforward amplifier in perfect balance with high accuracy, an automatic adjustment scheme using a pilot signal is set forth, for example, in the afore-mentioned Japanese Patent Publication Gazette No. 7-77330, and a device implementing the scheme is described in Toshio Nojima and Shoichi Narahashi, “Extremely Low-Distortion Multi-Carrier Amplifier for Mobile Communication Systems—Self-Adjusting Feed-Forward Amplifier (SAFF-A)—,” Institute of Electronics, Information and Communication Engineers of Japan, Radio Communications Systems Technical Report, RCS90-4, 1990.
A feedforward amplifier with improved stabilization and distortion compensation capabilities is disclosed, for example, in Japanese Patent Application Laid-Open Gazette No. 2000-353923. The feedforward amplifier is a modification of the prior art example of FIG. 1. As shown in FIG. 2, the variable attenuator 61, the variable phase shifter 62 and the delay line 64 of the first auxiliary amplifier distortion detection circuit in FIG. 1 are exchanged in position; a variable attenuator 55 and the a variable phase shifter 56 are provided immediately preceding the first amplifier distortion detection circuit 60; and a pilot signal introduced between stages of the main amplifier 14 is extracted by a directional coupler from the feedforward amplifier output (the output from the power combiner 53) and is detected by a pilot signal detector and controlled by a controller 97 so that its detected level is minimized.
In FIG. 1, the variable attenuator 61 and the variable phase shifter 62 on the input side of the first auxiliary amplifier 63 are shared by the distortion elimination circuit 50 and the first auxiliary amplifier distortion detection circuit 60. This presents a problem that even if the balance of the first auxiliary amplifier distortion detection circuit 60 is achieved by the variable attenuator 61 and the variable phase shifter 62, the balance of the distortion elimination circuit 50 cannot be achieved. Moreover, there is a problem that when the balance of the distortion elimination circuit 50 is achieved, the balance of the first auxiliary amplifier distortion detection circuit 60 cannot be reached.
Further, in such a modified feedforward amplifier as depicted in FIG. 2, when distortion occurs, the control range in the controller 97 becomes a fraction of the amplification factor of the first auxiliary amplifier 63 for the actual distortion range, making it impossible to stably control the pilot signal with high sensitivity.